• Owen
    21
    1, x=y defined: E!x & E!y & (All F)(Fx <-> Fy).
    and
    2. E!x defined: (Some F)(Fx).

    3. (All x)(x=x <-> E!x), is a theorem.
    If either or both do not exist then x=y is provably false.

    4. (All x)(x=x) is not valid.

    example
    (The present King if France)=(The present King if France), is false, even though
    (All F)(F(The present King if France) <-> F(The present King if France)), is tautologous.
  • Banno
    3.4k
    is it wrong to argue that a is not a because one a is on the left side of the copula and the other a is on the right side, and having different properties they are clearly not identical.jlrinc

    IS the sentence about the marks on either side of the "="?
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